Wafer producing method

ABSTRACT

A hexagonal single crystal wafer is produced from a hexagonal single crystal ingot. The depth of the focal point of a laser beam is gradually changed from a shallow position not reaching the depth corresponding to the desired thickness of the wafer to a deep position corresponding to the desired thickness of the wafer in such a manner that a parabola is described by the path of the focal point. When the spot area of the laser beam on the upper surface of the ingot becomes a predetermined maximum value, the deep position of the focal point is maintained.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wafer producing method for slicing ahexagonal single crystal ingot to produce a wafer.

Description of the Related Art

Various devices such as ICs and LSIs are formed by forming a functionallayer on the front side of a wafer formed of silicon or the like andpartitioning this functional layer into a plurality of regions along aplurality of crossing division lines. The division lines of the waferare processed by a processing apparatus such as a cutting apparatus anda laser processing apparatus to thereby divide the wafer into aplurality of individual device chips corresponding to the respectivedevices. The device chips thus obtained are widely used in variousequipment such as mobile phones and personal computers. Further, powerdevices or optical devices such as LEDs and LDs are formed by forming afunctional layer on the front side of a wafer formed of a hexagonalsingle crystal such as SiC and GaN and partitioning this functionallayer into a plurality of regions along a plurality of crossing divisionlines.

In general, the wafer on which the devices are to be formed is producedby slicing an ingot with a wire saw. Both sides of the wafer obtainedabove are polished to a mirror finish (see Japanese Patent Laid-open No.2000-94221, for example). This wire saw is configured in such a mannerthat a single wire such as a piano wire having a diameter ofapproximately 100 to 300 μm is wound around many grooves formed onusually two to four guide rollers to form a plurality of cuttingportions spaced in parallel with a given pitch. The wire is operated torun in one direction or opposite directions, thereby slicing the ingotinto a plurality of wafers.

However, when the ingot is cut by the wire saw and both sides of eachwafer are polished to obtain the product, 70 to 80% of the ingot isdiscarded to cause a problem of poor economy. In particular, a hexagonalsingle crystal ingot of SiC or GaN, for example, has high Mohs hardnessand it is therefore difficult to cut this ingot with the wire saw.Accordingly, considerable time is required for cutting of the ingot,causing a reduction in productivity. That is, there is a problem inefficiently producing a wafer in this prior art.

A technique for solving this problem is described in Japanese PatentLaid-open No. 2013-49161. This technique includes the steps of settingthe focal point of a laser beam having a transmission wavelength to SiCinside a hexagonal single crystal ingot, next applying the laser beam tothe ingot as scanning the laser beam on the ingot to thereby form amodified layer and cracks in a separation plane inside the ingot, andnext applying an external force to the ingot to thereby break the ingotalong the separation plane where the modified layer and the cracks areformed, thus separating a wafer from the ingot. In this technique, thelaser beam is scanned spirally or linearly along the separation plane sothat a first application point of the laser beam and a secondapplication point of the laser beam nearest to the first applicationpoint have a predetermined positional relation with each other. As aresult, the modified layer and the cracks are formed at very highdensity in the separation plane of the ingot.

SUMMARY OF THE INVENTION

However, in the ingot cutting method described in Japanese PatentLaid-open No. 2013-49161 mentioned above, the laser beam is scannedspirally or linearly on the ingot. In the case of linearly scanning thelaser beam, the direction of scanning of the laser beam is notspecified. In the ingot cutting method described in Japanese PatentLaid-open No. 2013-49161, the pitch (spacing) between the firstapplication point and the second application point of the laser beam asmentioned above is set to 1 to 10 μm. This pitch corresponds to thepitch of the cracks extending from the modified layer along a c-planedefined in the ingot.

In this manner, the pitch of the application points of the laser beam tobe applied to the ingot is very small. Accordingly, regardless ofwhether the laser beam is scanned spirally or linearly, the laser beammust be applied with a very small pitch and the improvement inproductivity is not yet sufficient.

It is therefore an object of the present invention to provide a waferproducing method which can efficiently produce a wafer from an ingot.

In accordance with an aspect of the present invention, there is provideda wafer producing method for producing a hexagonal single crystal waferfrom a hexagonal single crystal ingot having a first surface, a secondsurface opposite to the first surface, a c-axis extending from the firstsurface to the second surface, and a c-plane perpendicular to thec-axis, the wafer producing method including a separation start pointforming step of setting the focal point of a laser beam having atransmission wavelength to the ingot inside the ingot at a predetermineddepth from the first surface, which depth corresponds to the thicknessof the wafer to be produced, and next applying the laser beam to thefirst surface as relatively moving the focal point and the ingot tothereby form a modified layer parallel to the first surface and cracksextending from the modified layer along the c-plane, thus forming aseparation start point, and a wafer separating step of separating aplate-shaped member having a thickness corresponding to the thickness ofthe wafer from the ingot at the separation start point after performingthe separation start point forming step, thus producing the wafer fromthe ingot, the separation start point forming step including a modifiedlayer forming step of relatively moving the focal point of the laserbeam in a first direction perpendicular to a second direction where thec-axis is inclined by an off angle with respect to a normal to the firstsurface and the off angle is formed between the first surface and thec-plane, thereby linearly forming the modified layer extending in thefirst direction, and an indexing step of relatively moving the focalpoint in the second direction to thereby index the focal point by apredetermined amount, wherein in making the focal point of the laserbeam enter the ingot in the modified layer forming step, the depth ofthe focal point is gradually changed from a shallow position notreaching the depth corresponding to the thickness of the wafer to a deepposition corresponding to the thickness of the wafer in such a mannerthat a parabola is described by the path of the focal point, and whenthe spot area of the laser beam on the first surface becomes apredetermined maximum value, the deep position of the focal point ismaintained.

According to the wafer producing method of the present invention, eachmodified layer is formed at the predetermined depth from the firstsurface of the ingot and the cracks are formed on both sides of eachmodified layer so as to propagate along the c-plane. Accordingly, anyadjacent ones of the plural modified layers are connected togetherthrough the cracks formed therebetween, so that the plate-shaped memberhaving the thickness corresponding to the thickness of the wafer can beeasily separated from the ingot at the separation start point, thusproducing the hexagonal single crystal wafer from the ingot.Accordingly, the productivity can be sufficiently improved and theamount of the ingot to be discarded can be sufficiently reduced toapproximately 30%.

In forming a modified layer inside the ingot by applying a laser beamhaving a transmission wavelength to the ingot, it is known that themodified layer is formed at a position above the focal point of thelaser beam where the power density (average power/spot area) of thelaser beam becomes 6.8×10⁴ W/mm².

Accordingly, at the initial stage where the focal point of the laserbeam has entered the ingot from the outer circumference thereof, thepower density becomes 6.8×10⁴ W/mm² at the focal point, and the modifiedlayer is therefore formed at the focal point. Thereafter, the spot areaof the laser beam on the first surface of the ingot is graduallyincreased, so that the power density becomes 6.8×10⁴ W/mm² at a positionabove the focal point. That is, the modified layer is formed at thisposition above the focal point. Accordingly, the path of the modifiedlayer describes a parabola from a position far from the first surfacetoward the first surface. When the path of the modified layer reaches apredetermined position at a given depth from the first surface, thedepth of the modified layer becomes stable. Accordingly, in theconventional applying method for the laser beam, the plural elements ofthe linear modified layer cannot be formed in the same plane.

To solve this problem, the modified layer forming step in the presentinvention is improved in the following manner. At the initial stagewhere the focal point of the laser beam has entered the ingot, the depthof the focal point is gradually changed from a shallow position notreaching the depth corresponding to the thickness of the wafer to a deepposition corresponding to the thickness of the wafer in such a mannerthat a parabola as canceling the above parabola described by the path ofthe modified layer is described by the path of the focal point, and whenthe spot area of the laser beam on the first surface of the ingotbecomes a predetermined maximum value, the deep position of the focalpoint is maintained. Accordingly, the plural elements of the linearmodified layer can be formed in the same plane.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser processing apparatus suitablefor use in performing a wafer producing method of the present invention;

FIG. 2 is a block diagram of a laser beam generating unit;

FIG. 3A is a perspective view of a hexagonal single crystal ingot;

FIG. 3B is an elevational view of the ingot shown in FIG. 3A;

FIG. 4 is a perspective view for illustrating a separation start pointforming step;

FIG. 5 is a plan view of the ingot shown in FIG. 3A;

FIG. 6 is a schematic sectional view for illustrating a modified layerforming step;

FIG. 7 is a schematic plan view for illustrating the modified layerforming step;

FIGS. 8A and 8B are schematic sectional views for illustrating aconventional positioning method for the focal point of a laser beam;

FIGS. 9A and 9B are schematic sectional views for illustrating apositioning method for the focal point of a laser beam according to thepresent invention;

FIGS. 10A and 10B are perspective views for illustrating a waferseparating step; and

FIG. 11 is a perspective view of a hexagonal single crystal waferproduced from the ingot.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described indetail with reference to the drawings. Referring to FIG. 1, there isshown a perspective view of a laser processing apparatus 2 suitable foruse in performing a wafer producing method of the present invention. Thelaser processing apparatus 2 includes a stationary base 4 and a firstslide block 6 mounted on the stationary base 4 so as to be movable inthe X direction. The first slide block 6 is moved in a feedingdirection, or in the X direction along a pair of guide rails 14 by afeeding mechanism 12 composed of a ball screw 8 and a pulse motor 10.

A second slide block 16 is mounted on the first slide block 6 so as tobe movable in the Y direction. The second slide block 16 is moved in anindexing direction, or in the Y direction along a pair of guide rails 24by an indexing mechanism 22 composed of a ball screw 18 and a pulsemotor 20. A support table 26 is mounted on the second slide block 16.The support table 26 is movable in the X direction and the Y directionby the feeding mechanism 12 and the indexing mechanism 22 and alsorotatable by a motor stored in the second slide block 16.

A column 28 is provided on the stationary base 4 so as to project upwardtherefrom. A laser beam applying mechanism (laser beam applying means)30 is mounted on the column 28. The laser beam applying mechanism 30 iscomposed of a casing 32, a laser beam generating unit 34 (see FIG. 2)stored in the casing 32, and focusing means (laser head) 36 mounted onthe front end of the casing 32. An imaging unit 38 having a microscopeand a camera is also mounted on the front end of the casing 32 so as tobe aligned with the focusing means 36 in the X direction.

As shown in FIG. 2, the laser beam generating unit 34 includes a laseroscillator 40 for generating a pulsed laser beam such as YAG laser andYVO4 laser, repetition frequency setting means 42 for setting therepetition frequency of the pulsed laser beam to be generated from thelaser oscillator 40, pulse width adjusting means 44 for adjusting thepulse width of the pulsed laser beam to be generated from the laseroscillator 40, and power adjusting means 46 for adjusting the power ofthe pulsed laser generated from the laser oscillator 40. Althoughespecially not shown, the laser oscillator 40 has a Brewster window, sothat the laser beam generated from the laser oscillator 40 is a laserbeam of linearly polarized light. After the power of the pulsed laserbeam is adjusted to a predetermined power by the power adjusting means46 of the laser beam generating unit 34, the pulsed laser beam isreflected by a mirror 48 included in the focusing means 36 and nextfocused by a focusing lens 50 included in the focusing means 36. Thefocusing lens 50 is positioned so that the pulsed laser beam is focusedinside a hexagonal single crystal ingot 11 as a workpiece fixed to thesupport table 26.

Referring to FIG. 3A, there is shown a perspective view of the hexagonalsingle crystal ingot 11 as a workpiece to be processed. FIG. 3B is anelevational view of the hexagonal single crystal ingot 11 shown in FIG.3A. The hexagonal single crystal ingot (which will be hereinafterreferred to also simply as ingot) 11 is selected from a SiC singlecrystal ingot or a GaN single crystal ingot. The ingot 11 has a firstsurface (upper surface) 11 a and a second surface (lower surface) 11 bopposite to the first surface 11 a. The first surface 11 a of the ingot11 is preliminarily polished to a mirror finish because the laser beamis applied to the first surface 11 a.

The ingot 11 has a first orientation flat 13 and a second orientationflat 15 perpendicular to the first orientation flat 13. The length ofthe first orientation flat 13 is set greater than the length of thesecond orientation flat 15. The ingot 11 has a c-axis 19 inclined by anoff angle α toward the second orientation flat 15 with respect to anormal 17 to the upper surface 11 a and also has a c-plane 21perpendicular to the c-axis 19. The c-plane 21 is inclined by the offangle α with respect to the upper surface 11 a. In general, in thehexagonal single crystal ingot 11, the direction perpendicular to thedirection of extension of the shorter second orientation flat 15 is thedirection of inclination of the c-axis 19. The c-plane 21 is set in theingot 11 innumerably at the molecular level of the ingot 11. In thispreferred embodiment, the off angle α is set to 4°. However, the offangle α is not limited to 4° in the present invention. For example, theoff angle α may be freely set in the range of 1° to 6° in manufacturingthe ingot 11.

Referring again to FIG. 1, a column 52 is fixed to the left side of thestationary base 4. The column 52 is formed with a vertically elongatedopening 53, and a pressing mechanism 54 is vertically movably mounted tothe column 52 so as to project from the opening 53.

As shown in FIG. 4, the ingot 11 is fixed to the upper surface of thesupport table 26 by using a wax or adhesive in the condition where thesecond orientation flat 15 of the ingot 11 becomes parallel to the Xdirection. In other words, as shown in FIG. 5, the direction offormation of the off angle α is shown by an arrow Y1. That is, thedirection of the arrow Y1 is the direction where the intersection 19 abetween the c-axis 19 and the upper surface 11 a of the ingot 11 ispresent with respect to the normal 17 to the upper surface 11 a.Further, the direction perpendicular to the direction of the arrow Y1 isshown by an arrow A. Then, the ingot 11 is fixed to the support table 26in the condition where the direction of the arrow A becomes parallel tothe X direction.

Accordingly, the laser beam is scanned in the direction of the arrow Aperpendicular to the direction of the arrow Y1, or the direction offormation of the off angle α. In other words, the direction of the arrowA perpendicular to the direction of the arrow Y1 where the off angle αis formed is defined as the feeding direction of the support table 26.

In the wafer producing method of the present invention, it is importantthat the scanning direction of the laser beam to be applied from thefocusing means 36 is set to the direction of the arrow A perpendicularto the direction of the arrow Y1 where the off angle α of the ingot 11is formed. That is, it was found that by setting the scanning directionof the laser beam to the direction of the arrow A as mentioned above inthe wafer producing method of the present invention, cracks propagatingfrom a modified layer formed inside the ingot 11 by the laser beamextend very long along the c-plane 21.

In performing the wafer producing method according to this preferredembodiment, a separation start point forming step is performed in such amanner that the focal point of the laser beam having a transmissionwavelength (e.g., 1064 nm) to the hexagonal single crystal ingot 11fixed to the support table 26 is set inside the ingot 11 at apredetermined depth from the first surface (upper surface) 11 a, whichdepth corresponds to the thickness of a wafer to be produced, and thelaser beam is next applied to the upper surface 11 a as relativelymoving the focal point and the ingot 11 to thereby form a modified layer23 parallel to the upper surface 11 a and cracks 25 propagating from themodified layer 23 along the c-plane 21, thus forming a separation startpoint (separation plane) where the modified layer 23 and the cracks 25are formed.

This separation start point forming step includes a modified layerforming step of relatively moving the focal point of the laser beam inthe direction of the arrow A perpendicular to the direction of the arrowY1 where the c-axis 19 is inclined by the off angle α with respect tothe normal 17 to the upper surface 11 a and the off angle α is formedbetween the c-plane 21 and the upper surface 11 a, thereby forming themodified layer 23 inside the ingot 11 and the cracks 25 propagating fromthe modified layer 23 along the c-plane 21, and also includes anindexing step of relatively moving the focal point in the direction offormation of the off angle α, i.e., in the Y direction to thereby indexthe focal point by a predetermined amount as shown in FIG. 7 and FIGS.8A and 8B.

As shown in FIGS. 6 and 7, the modified layer 23 is linearly formed soas to extend in the X direction, so that the cracks 25 propagate fromthe modified layer 23 in opposite directions along the c-plane 21. Inthe wafer producing method according to this preferred embodiment, theseparation start point forming step further includes an index amountsetting step of measuring the width of the cracks 25 formed on one sideof the modified layer 23 along the c-plane 21 and then setting the indexamount of the focal point according to the width measured above. Morespecifically, letting W1 denote the width of the cracks 25 formed on oneside of the modified layer 23 so as to propagate from the modified layer23 along the c-plane 21, the index amount W2 of the focal point is setin the range of W1 to 2W1.

For example, the separation start point forming step is performed underthe following laser processing conditions.

Light source: Nd:YAG pulsed laser

Wavelength: 1064 nm

Repetition frequency: 80 kHz

Average power: 3.2 W

Pulse width: 4 ns

Spot diameter: 3 μm

Numerical aperture (NA) of the focusing lens: 0.43

Index amount: 250 to 400 μm

Work feed speed: 120 to 260 mm/second

In the laser processing conditions mentioned above, the width W1 of thecracks 25 propagating from the modified layer 23 along the c-plane 21 inone direction as viewed in FIG. 6 is set to approximately 250 μm, andthe index amount W2 is set to 400 μm. However, the average power of thelaser beam is not limited to 3.2 W. When the average power of the laserbeam was set to 2 W to 4.5 W, good results were obtained in thepreferred embodiment. In the case that the average power was set to 2 W,the width W1 of the cracks 25 was approximately 100 μm. In the case thatthe average power was set to 4.5 W, the width W1 of the cracks 25 wasapproximately 350 μm.

In the case that the average power is less than 2 W or greater than 4.5W, the modified layer 23 cannot be well formed inside the ingot 11.Accordingly, the average power of the laser beam to be applied ispreferably set in the range of 2 W to 4.5 W. For example, the averagepower of the laser beam to be applied to the ingot 11 was set to 3.2 Win this preferred embodiment. As shown in FIG. 6, the depth D1 of thefocal point from the upper surface 11 a in forming the modified layer 23was set to 500 μm.

Referring to FIG. 8A, there is shown a schematic sectional view forillustrating a conventional positioning method for the focal point of alaser beam. FIG. 8B is an enlarged view of a part shown in FIG. 8A. Itis known that a modified layer is formed inside a workpiece at aposition above the focal point of the laser beam where the power density(average power/spot area) of the laser beam becomes 6.8×10⁴ W/mm².Accordingly, at the initial stage where the focal point of the laserbeam has entered the ingot 11 from the outer circumference (sidesurface) thereof, the power density becomes 6.8×10⁴ W/mm² at the focalpoint P, and the modified layer 23 is therefore formed at the focalpoint P. Thereafter, the spot area of the laser beam on the firstsurface (upper surface) 11 a of the ingot 11 is gradually increased, sothat the power density becomes 6.8×10⁴ W/mm² at a position above thefocal point P. That is, the modified layer 23 is formed at this positionabove the focal point P. Accordingly, the path of the modified layer 23describes a parabola from a position far from the upper surface 11 atoward the upper surface 11 a. When the path of the modified layer 23reaches a predetermined position at a given depth from the upper surface11 a, the depth of the modified layer 23 becomes stable.

Letting H1 denote the height of this predetermined position from thefocal point P and D2 denote the distance from the side surface of theingot 11 to this predetermined position, H1=15 to 25 μm and D2=20 to 30μm. Accordingly, in the conventional forming method for the focal point,the plural elements of the linear modified layer 23 cannot be formed atthe same depth from the upper surface 11 a of the ingot 11, i.e., cannotbe formed in the same plane.

To solve this problem, the wafer producing method according to thepresent invention has improved the forming method for the focal point asshown in FIGS. 9A and 9B. In making the focal point P of the laser beamenter the ingot 11, the depth of the focal point P is gradually changedfrom a shallow position not reaching the depth corresponding to thethickness of the wafer to a deep position corresponding to the thicknessof the wafer in such a manner that a parabola is described by the pathof the focal point P, and when the spot area of the laser beam on thefirst surface (upper surface) 11 a becomes a predetermined maximumvalue, the deep position of the focal point P is maintained. Thevertical movement of the focal point P is controlled by the finemovement of the focusing means 36. By controlling the depth of the focalpoint P as shown in FIG. 9A, the plural elements of the linear modifiedlayer 23 can be formed in the same plane as shown in FIG. 9B.

In this manner, the focal point of the laser beam is sequentiallyindexed to form a plurality of modified layers 23 at the depth D1 in thewhole area of the ingot 11 and the cracks 25 extending from eachmodified layer 23 along the c-plane 21 as shown in FIGS. 6 and 7.Thereafter, a wafer separating step is performed in such a manner thatan external force is applied to the ingot 11 to thereby separate aplate-shaped member having a thickness corresponding to the thickness ofthe wafer to be produced, from the ingot 11 at the separation startpoint composed of the modified layers 23 and the cracks 25, thusproducing a hexagonal single crystal wafer 27 shown in FIG. 11.

This wafer separating step is performed by using the pressing mechanism54 shown in FIG. 1. The configuration of the pressing mechanism 54 isshown in FIGS. 10A and 10B. The pressing mechanism 54 includes a head 56vertically movable by a moving mechanism (not shown) incorporated in thecolumn 52 shown in FIG. 1 and a pressing member 58 rotatable in thedirection shown by an arrow R in FIG. 10B with respect to the head 56.As shown in FIG. 10A, the pressing mechanism 54 is relatively positionedabove the ingot 11 fixed to the support table 26. Thereafter, as shownin FIG. 10B, the head 56 is lowered until the pressing member 58 comesinto pressure contact with the upper surface 11 a of the ingot 11.

In the condition where the pressing member 58 is in pressure contactwith the upper surface 11 a of the ingot 11, the pressing member 58 isrotated in the direction of the arrow R to thereby generate a torsionalstress in the ingot 11. As a result, the ingot 11 is broken at theseparation start point where the modified layers 23 and the cracks 25are formed. Accordingly, the hexagonal single crystal wafer 27 shown inFIG. 11 can be separated from the hexagonal single crystal ingot 11.After separating the wafer 27 from the ingot 11, the separation surfaceof the wafer 27 and the separation surface of the ingot 11 arepreferably polished to a mirror finish.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer producing method for producing ahexagonal single crystal wafer from a hexagonal single crystal ingothaving a first surface, a second surface opposite to said first surface,a c-axis extending from said first surface to said second surface, and ac-plane perpendicular to said c-axis, said wafer producing methodcomprising: a separation start point forming step of setting the focalpoint of a laser beam having a transmission wavelength to said ingotinside said ingot at a predetermined depth from said first surface,which depth corresponds to the thickness of said wafer to be produced,and next applying said laser beam to said first surface as relativelymoving said focal point and said ingot to thereby form a modified layerparallel to said first surface and cracks extending from said modifiedlayer along said c-plane, thus forming a separation start point; and awafer separating step of separating a plate-shaped member having athickness corresponding to the thickness of said wafer from said ingotat said separation start point after performing said separation startpoint forming step, thus producing said wafer from said ingot, saidseparation start point forming step including a modified layer formingstep of relatively moving the focal point of said laser beam in a firstdirection perpendicular to a second direction where said c-axis isinclined by an off angle with respect to a normal to said first surfaceand said off angle is formed between said first surface and saidc-plane, thereby linearly forming said modified layer extending in saidfirst direction, and an indexing step of relatively moving said focalpoint in said second direction to thereby index said focal point by apredetermined amount, wherein in making the focal point of said laserbeam enter said ingot in said modified layer forming step, the depth ofsaid focal point is gradually changed from a shallow position notreaching the depth corresponding to the thickness of said wafer to adeep position corresponding to the thickness of said wafer in such amanner that a parabola is described by the path of said focal point, andwhen the spot area of said laser beam on said first surface becomes apredetermined maximum value, said deep position of said focal point ismaintained.
 2. The wafer producing method according to claim 1, whereinsaid hexagonal single crystal ingot is selected from the groupconsisting of an SiC single crystal ingot and a GaN single crystalingot.